Generally, a motor vehicle automatic transmission includes a number of gear elements coupling its input and output shafts, and a related number of torque establishing devices such as clutches and brakes which are selectively engageable to activate certain gear elements for establishing a desired speed ratio between the input and output shafts. The brake can be of the band type or disk type; engineering personnel in the automotive art refer to disc type brakes in transmissions as "clutches" or "reaction clutches". As used herein, the terms "clutches" and "torque transmitting devices" will be used to refer to brakes as well as clutches.
The input shaft is connected to the vehicle engine through a fluid coupling, such as a torque converter, and the output shaft is connected directly to the vehicle wheels. Shifting from one forward speed ratio to another is performed in response to engine throttle and vehicle speed, and generally involves releasing or disengaging the clutch (off-going) associated with the current speed ratio and applying or engaging the clutch (on-coming) associated with the desired speed ratio. Garage shifts require shifting from neutral to forward or reverse, or shifting between forward and reverse. When shifting from neutral, no off-going clutch is employed but an on-coming clutch is applied. For other garage shifts, a clutch is released and another is applied in a controlled manner.
The speed ratio is defined as the transmission input speed or turbine speed divided by the output speed. Thus, a low gear range has a high speed ratio and a higher gear range has a lower speed ratio. To perform an upshift, for example, a shift is made from a high speed ratio to a low speed ratio. In the type of transmission involved in this invention, the upshift is accomplished by disengaging a clutch associated with the higher speed ratio and engaging a clutch associated with the lower speed ratio to thereby reconfigure the gear set to operate at the lower speed ratio.
As used herein, the term "garage shifts" encompasses shifts at or near zero output speed including shifts from neutral and shifts between forward and reverse (direction changing shifts). Garage shifts also include relatively high speed shifts from neutral and direction changing shifts. Output speeds above 60 RPM are considered to be "high speeds" and garage shifts above a calibrated limit speed, such as 100 RPM are not permitted Although a wide variety of shift conditions are encountered in this group of shift types, it is desirable to consolidate them for management by a single control program with a minimum of special provisions for special cases.